1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a reflective type liquid crystal display device and fabricating method thereof, by which resolution is enhanced.
2. Discussion of the Related Art
Recently, a liquid crystal display (LCD) device, one of the noticeable flat panel display devices, controls optical anisotropy by applying an electric field to liquid crystals provided with fluidity of liquid and crystalline optical properties. Thus, an LCD includes features such as power consumption less than that of CRT (cathode ray tube), compact size, wide screen, high definition, and the like, and therefore have become very popular.
In the liquid crystal display device, an upper color filter substrate is assembled to a lower thin film transistor (TFT) array substrate facing each other. Liquid crystals having dielectric anisotropy are provided between the upper and lower substrates. The LCD is driven so that TFTs attached to several hundred-thousands pixels are switched via pixel selecting address lines to apply voltage to the corresponding pixels.
Meanwhile, a liquid crystal display device can be categorized into a transmissive type liquid crystal display device, which uses a backlight, a reflective type liquid crystal display device, which uses an external natural light instead of a backlight, and a transflective type liquid crystal display device overcoming the large power consumption problem of using the backlight of the transmissive type liquid crystal display device and the invisibility problem of the reflective liquid crystal display device in case of the insufficient external natural light.
The transflective type liquid crystal display device is provided with both reflective and transmissive parts and can be used as both a reflective type and a transmissive type.
Hence, the pixel electrode in an LCD device is a transmissive electrode or a reflective electrode depending on the type of liquid crystal display device. The transmissive electrode is provided to a transmissive part of the transmissive or transflective type liquid crystal display device, and the reflective electrode is provided to a reflective part of the reflective or transflective type liquid crystal display device.
The transmissive electrode of the transmissive or transflective type liquid crystal display device leads a light of a backlight, which is incident via a lower substrate, to a liquid crystal layer to increase brightness. The reflective electrode of the reflective or transflective liquid crystal display device reflects an external light, which is incident via an upper substrate, to increase brightness when an external natural light is sufficient.
A liquid crystal display device provided with a reflective electrode according to a related art is explained by referring to the attached drawings as follows.
FIG. 1 is a cross-sectional diagram of a reflective type liquid crystal display device according to a related art, and FIG. 2 is a perspective diagram of a reflective type liquid crystal display device according to a related art.
Referring to FIG. 1, in a liquid crystal display device, a gate line 12 and data line 15 cross each other on a lower substrate 11 to define a sub-pixel. A thin film transistor (TFT) is formed on a crossing between the gate and data lines 12 and 15. A pixel electrode 17 is formed in the sub-pixel to be electrically connected to the thin film transistor.
The thin film transistor (TFT) consists of a gate electrode 12a, a gate insulating layer 13, a semiconductor layer 14, and source/drain electrodes 15a/15b, which are stacked in order.
A black matrix layer 22 that blocks light at a periphery of the sub-pixel, a color filter layer 23 of R/G/B (red/green/blue) for implementing color of the sub-pixel, and a common electrode 24 for generating an electric field together with the pixel electrode 17 are formed on the upper substrate 21.
The upper and lower substrates 11 and 21 are assembled together to leave a predetermined gap between them, and a liquid crystal layer is provided in the gap between the upper and lower substrates 11 and 21.
A reflective type liquid crystal display device, the pixel electrode 17 is formed of a metal of high reflectivity such Al, Cu, and the like. In a transflective type liquid crystal display device, a reflective part and transmissive part are defined A reflective electrode is formed in the reflective part using metal of high reflectivity, whereas a transmissive electrode of a transparent conductive material is formed in the transmissive part to be connected to the reflective electrode.
A retardation film 54 and a polarizing plate 55 are further provided on the upper surface of the upper substrate 21 of the liquid crystal display device.
A method of fabricating a liquid crystal display device is explained as follows.
First of all, metal of low resistance is deposited on the lower substrate 11 by sputtering. A gate line (‘12’ in FIG. 2) and the gate electrode 12a are then formed by photolithography.
The gate insulating layer 13 is formed on an entire surface including the gate electrode 12a, and the semiconductor layer 14 is formed on the gate insulating layer 13 over the gate electrode 12a. 
Metal of low resistance is deposited again on an entire surface including the gate insulating layer 13. A data line (‘15’ in FIG. 2) and the source/drain electrodes 15a/15b are then formed by photolithography.
In doing so, the data line 15 is formed to cross with the gate line 12 to define the sub-pixel, and the source/drain electrodes 15a/15b are formed on the semiconductor layer 14 to complete the corresponding thin film transistor (TFT).
Subsequently, an organic or inorganic insulating material is deposited at a predetermined thickness on an entire surface including the thin film transistor (TFT) to form a protective layer 16. The protective layer 16 is partially removed to expose a predetermined portion of the drain electrode 15b of the thin film transistor. The pixel electrode 17 is formed on the protective layer 16 in the sub-pixel to be connected to the drain electrode 15b. In case of the reflective type liquid crystal display device, the pixel electrode 17 is provided by a reflective electrode of high reflectivity metal.
Thereafter, a metal layer of high reflectivity is deposited on the upper substrate 21. The metal layer is then patterned to remain on an edge of the sub-pixel only to form the black matrix 22. The color filter layer 23 having an order of red/green/blue is formed on the sub-pixel excluding the black matrix 22.
The color filter layer can be formed by dyeing, dispersion, coating, electrophoretic deposition, or the like, and more particularly, by pigment dispersion.
Specifically, a first color resist colored by red is coated to completely cover the black matrix 22. Exposure is performed on the first color resist using a mask. An unexposed portion of the first color resist is removed to form a first colored layer pattern.
Subsequently, the above steps are repeated to form second and third colored layer patterns to form the color filter layer 23 of R/G/B. The color filter layer 23, as illustrated in FIG. 2, is formed to have a uniform sequence of R/G/B.
In the above-configuration of the color filter layer 23, three sub-pixels consisting of R/G/B implement one pixel to represent colors.
In doing so, a negative resist is used as the color resist, whereby the unexposed portion can be removed. And, the mask used for the exposure of the first color resist is shifted to use in forming the second and third colored layer patterns.
A common electrode 25 for applying an electric field to a liquid crystal cell together with the reflective electrode 17 is formed on an entire surface including the color filter layer 23.
The upper and lower substrates 11 and 21 are assembled to oppose each other after a seal pattern (not shown in the drawing) has been formed on an edge of a display area of the upper or lower substrate 21 or 11 having various devices formed thereon. The liquid crystal layer 25 is then formed between the upper and lower substrates 21 and 11.
The retardation film 54 for changing a polarization of light is provided to the upper surface of the upper substrate 21. For example, the retardation film 54 converts a linearly polarized incident light to a circularly polarized light using a quarter wave plate (QWP) having a λ/4 phase difference, and vice versa.
A polarizing plate 55 converting a natural light to a linearly polarized light by transmitting a light parallel to an optical transmission axis only is arranged outside the retardation film 54.
Once an external natural light is incident on the liquid crystal display device, the incident natural light is passed through the polarizing plate 55 to be converted to the linearly polarized light. The converted linearly polarized light is then passed through the retardation film 54 to be converted to the circularly polarized light. Subsequently, the circularly polarized light is passed through the upper substrate 21, color filter layer 23, and common electrode 24, which have no influence on the phase of the circularly polarized light at all, in turn.
The circularly polarized light is then passed through the liquid crystal layer 25. In case that the liquid crystal layer 25 is formed to have a λ/4 phase difference, the circularly polarized light is converted to the linearly polarized light again. The linearly polarized light is reflected on the reflective electrode 17 to turn into the circularly polarized light via the liquid crystal layer 25. The circularly polarized light is passed through the retardation film 54 to turn into the linearly polarized light. The linearly polarized light is then passed through the polarizing plate 55. In doing so, if a polarized direction of the linearly polarized light coincides with the optical transmission axis, the corresponding light is entirely transmitted. If the polarized direction of the linearly polarized light is perpendicular to the optical transmission axis, no light is outputted.
Besides, when the light is outputted, all the colors except a target color of the color filter layer are absorbed in the color filter layer so that a specific color of R/G/B is projected only.
Meanwhile, a backlight unit (not shown in the drawing) may be provided to a backside of the liquid crystal display device to be used as a light source in transmissive mode.
The above-configured reflective type liquid crystal display device operates using an external light incident via the upper substrate, thereby reducing power consumption due to limited or no use of backlight.
However, as three sub-pixels construct one pixel, limitation is put on the related art reflective type liquid crystal display device in raising resolution.